International Journal of Engineering and Science Invention (IJESI)

International Journal of Engineering and Science Invention (IJESI) is an international journal intended for professionals and researchers in all fields of computer science and electronics. IJESI publishes research articles and reviews within the whole field Engineering Science and Technology, new teaching methods, assessment, validation and the impact of new technologies and it will continue to provide information on the latest trends and developments in this ever-expanding subject. The publications of papers are selected through double peer reviewed to ensure originality, relevance, and readability. The articles published in our journal can be accessed online

Transcript of "International Journal of Engineering and Science Invention (IJESI)"

1.
International Journal of Engineering Science Invention
ISSN (Online): 2319 – 6734, ISSN (Print): 2319 – 6726
www.ijesi.org Volume 2 Issue 12 ǁ December. 2013 ǁ PP.28-35
“Studies on Thickness dependent transition temperature and
Electrical resistivity of co-evaporated thin films of Ag-Te.”
U.P.Shinde
Dept. of Physics Art’s, Science & Commerce College, Surgana-422211 Dist- Nashik (M.S.), India
ABSTRACT: Thin films of Ag-Te compound of varying composition and thickness have been deposited
on glass substrates employing three temperature method. Electrical properties of the films were studied as
function of thickness, composition and temperature. The Ag-Te thin films of compositions with Ag >50 at. wt. %,
~50 at. wt. % and <50 at. wt. % has been studied for resistivity measurements at different temperatures and
thicknesses. The Ag-Te compound of Ag >50 at. wt. % shows temperature dependent phase change from
semiconducting to metallic. The activation energy has been calculated as function of thickness, composition of
Ag-Te films. Thickness dependent transition temperature shows increase of thickness decreases transition
temperature.
KEYWORDS: Ag-Te, thin films, substrate, transition temperature, activation energy, resistivity.
I. INTRODUCTION
The Silver–Telluride is a I-VI compound of narrow band gap semiconductor shows first order phase
transition around 420 0K from semiconducting orthorhombic structure to metallic, cubic (fcc) structure. The
phase transition temperatures, as observed during the above semiconductor to metal transition of Ag 2Te reported
[1]. The dc conductivity of thermally evaporated Ag10Te90 and Ag20Te80 thin films has been studied.
Measurement of the temperature dependence of the electrical conductivity indicates that the increase of
annealing temperature leads to decrease of activation energy [2].
The electrical and structural properties of silver telluride films having stoichiometric composition have
been measured as a function of temperature [3, 4]. However these compounds are less investigated in the form
of thin films of different composition and thicknesses. From this point of view and considering application in
electronic devices the effect of transition temperature, thickness and composition of Ag-Te films are discussed.
We report the measurement of thickness dependent transition temperature, electrical properties such as
resistivity and activation energy of Ag-Te thin films of different composition and thicknesses.
II. EXPERIMENTAL DETAILS
Thin films of Ag-Te for the measurement of electrical resistivity were prepared by the three temperature method
[5-10]. Ag-Te films of different compositions and different thicknesses were prepared by vacuum deposition of the
constituent elements Ag ( 99.999% pure ) and Te (99.99% pure). Silver metal and tellurium powder were evaporated
from two different preheated conical mica baskets which in turn heated externally by nichrome wire. The films were
prepared on glass substrate kept at room temperature in a vacuum of the order of 10 –5 torr with an IBP TORR-120
vacuum unit. After adjusting the flux rates from two sources by varying the source current, films of varying
thicknesses and compositions were obtained by overcoming
the experimental difficulties in adjusting and
maintaining evaporation rates of the individual components. The films obtained were annealed at ~ 423 0K for 6 to 8
hours for the purpose of uniform distribution of the components of the deposits. The method employed to determine the
composition of the film were similar to those reported earlier [7-9]. The composition of Ag from Ag-Te films was
determined by employing absorption spectroscopy [11] at 350 nm.
The film thickness (d) was measured by multiple beam interferrometry [12] and gravimetric method [710] using the relation,
d 
M
g  A
cm
where
A = surface area of the film
M = Mass of the film
g = the density of the film material = x1 g1 + x2 g2
where g1, g2 and x1, x2 are densities and atomic fractions of Ag and Te elements respectively.
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Studies on Thickness dependent transition…
III. RESULTS AND DISCUSSION
The resistance of Ag-Te deposits having different compositions and thicknesses have been measured as
a function of temperature and the activation energy was calculated. The temperature of measurement varied
from 271 to 453 0K.
Ag67Te33
d ~ 3995 Ǻ
Log RH
3.6
Log R
3.4
Log RC
3.2
3
2.8
260
360
460
560
0
Tem perature, K
Fig.1: Plot of Log R verses temperature of Ag-Te films of (Ag) > 54 at. wt.% , from 271 to 453 0K.
Fig.1 shows the variation of Log R with temperature for Ag-Te deposits in the heating and cooling
cycles of [Ag] > 54 at.wt. % in the entire temperature range. Similar plots are obtained irrespective of thickness
of the deposits. It is seen that electrical resistance decreases with increasing temperature up to a certain
temperature, known as transformation temperature exhibits negative temperature coefficient (NTC ) of
resistance in conformity with semiconducting nature, and then it increases sharply with temperature at an above
the transition point exhibits a metallic behavior. During cooling cycle a reversible transformation from metallic
to semiconducting behavior is observed. The transition temperature lies between 406 to 421 0K depending on
thickness (d ) and composition of Ag-Te thin films. Similar observations have been reported on Ag2Te thin
films by Damodara Das and Karunakaran [13, 14].
Transition temp.( T ) 0K
From figure-1. It is seen that the temperature of transition during heating is higher than that of cooling,
probably due to removal of defects during heating of the specimen.
424
420
416
.
412
408
404
1500
2500
3500
4500
5500
6500
Thickness (d), Å
Fig.2: Plot of transition temperature (T) 0K verses thickness (d) Å, of Ag67Te33 films
Fig.2 reveals that as the thickness of the deposits increases the transition temperature decreases. This
decrease of transition temperature of Ag67Te33 films is attributed to decrease of defects with increase of
thickness.
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3.
Activation Energy (∆E), eV
Studies on Thickness dependent transition…
0.25
Ag22Te78
0.2
Ag67Te33
0.15
0.1
0.05
0
1500 3500 5500 7500
Thickness (d) , Å
Fig. 3: Plot of activation energy (∆E) eV verses thickness (d) Å, of Ag-Te films in the temperature
range of 271 to 453 0K.
Fig 3 shows variation of activation energy (∆E) as a function of thickness (d) and composition of the
films. It is seen that „∆E‟ decreases with increase of film thickness irrespective of compositions.
Fig. 4: Plot of activation energy (∆E) eV verses 1/d2 for semiconducting Ag-Te films in the temperature
range of 271 to 453 0K.
The observation of “band gap change” as a function of thickness can be due to any one combined
effects of the following causes as given by Damodara Das and Karunakaran [13]:
1) The change in the barrier height due to the size of the grains
in a polycrystalline films;
2) A large density of dislocations.
3) Quantum size effect; and
4) Change in stoichiometry.
We will consider the two possibilities, Slater [15] proposed that energy barriers are associated with grain
boundaries and their barrier heights can vary because of the charge accumulation at the boundaries. According
to him, the increased barrier height is given by
∆E=∆E0 + C (X-fD)2
--------------------- (1)
Where „∆E0‟ is the original barrier height, C is a term depending on the density of charge carriers, electronic
charge and dielectric constant of the material, X is the barrier width of about 200-300 Å, „D‟ is the
dimension of the grain, and „f‟ is a fraction of the order of 1/15 to 1/50 depending upon the charge
accumulation, and the carrier concentration .It is known from literature [16] that the grain size is approximately
proportional to thickness, and hence increases as thickness increases. Hence if we replace D, the grain size, in
the above expression by the film thickness (d) , we find that „∆E‟ should be proportional to (X-fd)2 .
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Studies on Thickness dependent transition…
However, in the present observations, we find that the band gap varies inversely as the square of the
thickness of the film and hence it can be concluded that the observed band gap variation with thickness can be
attributed to the above effect.
It is also known that a fairly large number of dislocations are created during the formation of the films
and their density increases as the thickness up to a particular thickness, beyond which the density is practically
constant. However, the dependence of the dislocation density on thickness has not been quantified and in any
case the dependence is complex. It has been suggested [17] that when the dislocation density is fairly high in the
case of thin films, there is an increase in the band gap of semiconductor material, because of the presence of
dislocations, provided that the dislocations are separated by a distance greater than the inter atomic distance.
This separation arises because of the alternate dilation and compression of the lattice on two sides of the
dislocation. However, the exact relationship between the dislocation density and /or their separation distance
and the change in the band gap has not been established and the magnitude of the separation caused is also
unknown. It is however clear that because of the complex nature of the dependence of the dislocation density
and the spacing, the variation of band gap with thickness due to above effect will be complex.
As in the present observation we observe a linear „∆E‟ verses 1/d2 dependence of the band gap
which is expected from the quantum size effect, it is reasonable to disregard the above possibility also.
Sandomirskii [18,19] was the first to consider quantum size effects in semimetallic and
semiconducting thin films, and to show that when the thickness of a thin films of any material is of the order of
de Broglie wavelength of conduction electrons / holes and is much less than the mean free path of the charge
carriers, the material exhibits quantum size effects. In the above situation, the electron / hole momentum
component normal to the film is quantized and as a consequence the contribution of motion normal to the film to
the kinetic energy of the charge carriers is also quantized and is given by
Ez = ( ћ2 π2/2m*d2 ) n2
-------------------- (2)
n = 1, 2, 3,----- etc
where ћ=h/2π,
„h‟ is the plank‟s constant, m* is the effective mass of the charge carrier, and „d‟ is the
thickness of the film. „E‟ is the kinetic energy contribution due to motion normal to the film plane.
As the lowest momentum component state is non zero, the lowest energy state for this motion is also
non zero. As a consequence, the bottom of the conduction band (zero state energy of electrons) and the top of
the valence band (zero state energy of holes) in the film state will shift to higher energies with respect to their
positions in the bulk state. As a result, if the material is semiconducting in the bulk state, its energy band gap
will be increased by an additional amount depending on the thickness of the film. As the increased separation is
a function of square of inverse thickness, a plot of band gap verses 1/ d2 will be linear, as has been observed in
the present case.
Gottlieb et al [20] reported the data of effective mass of electrons in Ag 2Te as ~0.05m0 and Fermi energy is of
the order of 10-2 eV. Hence the de-Broglie wavelength of electrons in Ag2Te will be of the order of 1000 Å, and
has high mobility (over 10000 cm2 / V Sec at room temperature).
Damodara Das and Karunakaran [13] also showed that the effective mass of electrons is 0.002 mo, and
largest wavelength found to be 3000 Å. From the slope of the band gap verses 1/d2 plot, m*e is calculated
using the relation
ћ2 π2/2m*e = slope,
--------------------------
(3)
assuming „mh’ to be very large. The largest de Broglie wavelength of the electrons calculated using the
relation
λ = h/p
p = ( 2m*e EF )1/2
------------------ (4)
---------------------------
(5)
where „p‟ is the momentum, m*e is effective mass of electrons and EF the Fermi energy of the electrons.
Taking Ef to be half the band gap of the film of largest thickness.
In the present study of Ag-Te films of different thicknesses and compositions, it is found that Ag 67Te33
shows n-type semiconducting nature. [Ag] > 54 at.wt.% of Ag in Ag-Te is n-type, thus the effective mass of
electrons in Ag67Te33 films is 0.0001035 m0 , and the de Broglie wavelength corresponding to half the band gap
(∆E) of the film of largest thickness is found using the equations 4 and 5,
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5.
Studies on Thickness dependent transition…
EF = ∆E
we get
P= 0.85117* 10-27
hence, λ ~ 7780 Å
Similar observations are also made for p-type Ag-Te films of [Ag] < 54 at.wt.% of Ag, thus for Ag22Te78,
the effective mass of holes (m*h ~ 2.5m*e),
P= 1.262* 10-27 hence,
λ ~ 5248 Å
(∆E) eV
The assumption made that m*h is always greater than m*e is reasonable and justified in the light of the
fact that as reported in the literature [20,3,21]. so that the conductivity observed is large due to electrons in AgTe films of [Ag] > 54 at. wt.% and smaller conductivity observed due to holes in Ag-Te films of [Ag] < 54 at.
wt.% i.e. in p-type material.
Considering the fourth possibility i.e. stoichiometric change influencing the band gap, it is noted that
there is every possibility of inhomogenity or non-stoichiometry in the Ag-Te film studied. In the present
investigation [Ag] / [Te] ratio is variable to a greater extent and the effect of dissolved „Ag‟ strongly influences
the band gap determination. Hence for films of different thicknesses with almost constant composition studied
in the same temperature range, the observed variation in „∆E‟ may be due to the effect of variable amounts of
dissolved silver getting locked in interstitial sites.
0.2
0.15
0.1
0.05
0
d~2650 Å
d~5100 Å
0
50
100
At. wt.%, Ag
Fig. 5: Plot of activation energy (∆E) eV verses at.wt. % of Ag in Ag-Te films in the temperature range of
271 to 453 0K
Fig.5 shows the variation of „∆E‟ with at. wt.% of Ag in Ag-Te deposits over entire temperature range.
It shows that „∆E‟ continuously decreases with increase of „Ag‟ content.
This probably is due to the increase of interstitial sites occupied by „Ag‟ which results in decrease of
„∆E‟ with increase of „Ag‟ content.
Ag-Te films containing [Ag] > 54 at. wt.% show metallic behavior ( Fig.1) in high temperature region.
An attempt has been made to fit our observation of metallic behavior to Fuchs-Sondheimer [22,23] and Mayadas
and Schatzkes [24] theories. The electrical resistivity of the metallic films can be expressed by FS model as
 = o [ 1+ (3lo / 8d) (1-p)]
-------------- (6)
Where „‟ is the electrical resistivity of the thin film, „o‟ the electrical resistivity of the bulk material,
„lo‟ the electron mean free path, „d‟ the film thickness and p the specularity parameter.
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Studies on Thickness dependent transition…
Assuming p=0 for total diffuse scattering, we obtained the value of lo ~9900 Å, in Ag-Te films.
Assuming that the values of o and lo(1-p) derived from „FS‟ model are also appropriate for the MS model.
Equation (8) has been used for estimating the values of ά, Negative values of ά suggest that MS model is not
valid for these films. Higher resistivity in thinner films may be related to smaller crystallite size and larger
structural defects.
The effect of thickness on resistivity () of film can also be explained in terms of Mathiessen rule
total = ideal + residual + thickness
-------------(9)
Where
ideal depends on the amplitude of the thermal motion of ions, residual is the component strongly
dependent on the lattice defects, but independent of temperature as long as these lattice defects are not affected
by temperature change, and thickness , the component of total that depends on the thickness of the film.
Now as the thickness of the film increases, effect of island structure, quantum size effect and effect
like grain size etc. are diminished, thus reducing the contribution of thickness on total . Contribution to total by
ideal and residual being characteristics of material may be taken as same for all thicknesses.
Thus total of the film decreases with the increasing film thickness.
1
d~3100 Å
-1
ρ*10 , Ω cm
0.8
d~5400 Å
0.6
0.4
0.2
0
0
20
40
60
80
At. w t.%, Ag
Fig.8: Plot of resistivity () verses At. wt.% of Ag in Ag-Te films in the temperature range 301 to 4530K
The variation of resistivity () with composition of Ag-Te deposits is shown in fig. 8 It has been
observed that resistivity () decreases with increase of „Ag‟ concentration in Ag-Te films. The decrease of ()
with increase of „Ag‟ concentration may be ascribed to inhomogeneous addition of Ag atoms.
IV. CONCLUSIONS:
Temperature dependent resistivity Shows semiconducting and metallic phase change above 54 At. wt.
% of Ag. Thickness and composition dependent activation energy shows inverse relation between them.
Thickness and composition dependent resistivity shows also The maximum photocurrent was developed at
about thickness of 500nm irrespective of composition and also stoichiometric ZnTe films irrespective of
thickness at room temperature.
V. ACKNOWLEDGEMENT
The author greatly thanks to the management authorities of M. G. Vidyamandir, the Principal, M. S
.G. College Malegaon Camp and the Principal, Arts Science and Commerce College Surgana for giving
valuable support to do this work.
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